The intake and exhaust valves (collectively referred to as “cylinder valves”) of the cylinders of an internal combustion engine are generally controlled by a camshaft. As known, the camshaft is a shaft rotating around a rotation axis and provided with a plurality of cams. Each cam includes one or more lobes protruding from a base circle. Generally, each cylinder valve is coupled to one of the cam of the camshaft, in a cam-follower configuration, so that the rotational movement of the camshaft is transformed into a reciprocating movement of the cylinder valves. In more detail, a cylinder valve is provided with a cam follower that engages a relevant cam of the camshaft, so that, during rotation of the cam, the cam follower is alternatively coupled to the base circle or to the lobe(s) of the cam, thus resulting in a reciprocating movement of the cylinder valve. In such a configuration, the movement of the cylinder valve is the same at every rotation of the camshaft.
However, it may be useful to change the activation of the cylinder valves, and in particular the extent of the lift of the cylinder valve. As an example, in gasoline engines, it is useful to reduce the intake valve lift according to the torque and power demand of the engine (e.g. low intake lift for the low loads and full lift for the partial load).
Various systems are known in the art that achieve this purpose, thus providing a variable valve actuation. One of these systems is known as a cam shifting system, wherein the camshaft is provided with a shifting unit (also known as sliding unit), which is rotatably fixed to the camshaft and slidable along the camshaft rotation axis, i.e. in the direction of extension of the camshaft. The shifting unit is provided with two or more cams preferably controlling each cam follower, and thus each cylinder valve of the internal combustion engine.
Some cams of the shifting unit are provided adjacent one to another so that a cylinder valve, and in particular its cam follower, can be alternatively engaged to different cams. In other words, it is possible to change the cam to which the cam follower, and thus the cylinder valve, is engaged. These different cams have the same base circle, but different distribution and/or configuration of the lobe(s), so that changing the cam to which a cam follower is engaged changes the operation of the cam follower itself.
To carry out the above mentioned changing, the shifting unit is movable along the camshaft, so that the cam follower of the cylinder valve (i.e. the portion of the cylinder valve coupled to the cam) contacts the external surface of a first cam. Due to the movement of the shifting unit in a direction parallel to the rotation axis of the camshaft, the cam follower reaches a position of engagement with the external surface of a second cam. The movement of the shifting unit along the camshaft is known as “shifting movement”. This expression will be used herein to indicate the movement of the shifting unit with respect to the camshaft, preferably along the camshaft rotation axis, i.e. along a direction that is parallel, or coincident, with the camshaft rotation axis. The shifting movement of the shifting unit is usually controlled by a driving pin, that engages one or more grooves arranged on the external surface of the shifting unit.
The arc of the external surface of the cams contacted by the cam follower during the shifting movement of the shifting unit must avoid steps. As a result, it is known to carry out the shifting movement of the shifting unit, when the cam follower can be moved from the base circle of a first cam to the base circle of a second cam, so that the cam follower does not encounter an obstacle during the passage from the first cam to a second cam. As a result, considering a 360 rotation degrees of the camshaft, the shifting movement of the shifting unit can be performed only during certain arcs of this rotation.
If these arcs are too short, the available time to carry out the shifting movement is also short, so that the required accelerations would be too high and it would not be possible to use the cam shifting system involving the movement of the shifting unit. This is particularly true when the cams are provided with a plurality of lobes, providing a further reduction of the base circle of the cams available to carry out the shifting movement of the shifting unit.
Accordingly, there is a need to solve the aforementioned drawbacks of the prior art. In particular, there is a need to provide an internal combustion engine including a cam shifting system having an extended time for carrying out the shifting movement of the shifting unit with respect to the known systems.